Split bridge circuit force sensor

ABSTRACT

A force sensor comprising a beam having a longitudinal axis and a proximal end portion and a distal end portion; a first Wheatstone bridge disposed on a first face of the beam, including multiple tension gauge resistors and multiple compression gauge resistors; a second Wheatstone bridge disposed on the first face of the beam, including multiple tension gauge resistors and multiple compression gauge resistors; wherein at least one tension gauge resistor and at least one compression gauge resistor from each of the first and second Wheatstone bridges is disposed at a proximal end portion of the beam; wherein at least one tension gauge resistor and at least one compression gauge resistor from each of the first and second Wheatstone bridges is disposed at a distal end portion of the beam.

CLAIM OF PRIORITY

This application is a U.S. National Stage Filing under 35 U.S.C. 371from International Application No. PCT/US2018/061113, filed Nov. 14,2018, and published as WO 2019/099562 A1 on May 23, 2019, which claimsthe benefit of priority to U.S. Provisional Patent Application Ser. No.62/586,721, filed on Nov. 15, 2017, and to U.S. Provisional PatentApplication Ser. No. 62/586,166, filed on Nov. 14, 2017, each of whichis incorporated by reference herein in its entirety.

BACKGROUND

Force sensing and feedback during a minimally invasive surgicalprocedure may bring better immersion, realism and intuitiveness to asurgeon performing the procedure. For the best performance of hapticsrendering and accuracy, force sensors may be placed on a surgicalinstrument and as close to the anatomical tissue interaction aspossible. One approach is to embed a force sensor at a distal end of asurgical instrument shaft with electrical strain gauges formed on theforce transducer, through printing or additive deposition processes, tomeasure strain imparted to the surgical instrument.

FIG. 1 is an illustrative drawing representing a force sensor thatincludes a rectangular beam with four full-Wheatstone bridges(full-bridges). A bridge circuit is a circuit topology of electricalcircuit in which two circuit branches (usually in parallel with eachother) are bridged by a third branch connected between the first twobranches at some intermediate point along them. Two full-bridges areformed on each of two adjacent orthogonal side faces of the beam tomeasure forces orthogonal to a longitudinal axis of the beam. The beammay be secured to a distal portion of a surgical instrument shaft tosense forces orthogonal to a longitudinal axis of the shaft. A forcesapplied orthogonal to a side face of the beam (i.e. an X or Y force) isdetermined by subtracting force measurements determined by thefull-bridges at proximal and distal end portions of that side face ofthe beam.

A force sensor may experience a variety of different strain sourcesincluding: the orthogonal force of interest to be measured, moment, offaxis force, off axis moment, compression/tension, torsion, ambienttemperature and gradient temperature. Each of the full-bridges cancelsthe following stress: temperature, torsion, off axis force, and off axismoment. Each individual full-bridge output indicates stress due toforce, moment, and compression/tension. The subtraction of an outputvalue produced by a proximal full-bridge formed on a side face from anoutput value produced by a distal full-bridge on the same side face,cancels the moment and compression/tension, resulting in an output valuethat represents the orthogonal force of interest to be measured.

A surgical instrument force sensor may be critical to ensuring patientsafety. Accordingly, force sensor error detection may be required toprotect against harm by detecting force sensor failures. One approach toerror detection may be to provide additional full-bridges to produceredundant force measurements that can be compared to detect errors.However, limited space on beam side faces makes formation of additionalfull-bridges on a side face impractical. Moreover, a manufacturingprocess typically is limited to formation of bridges at most on two sidefaces. Formation of bridges on four side faces would increasemanufacturing cost significantly.

SUMMARY

In one aspect, a force sensor includes a beam having a longitudinal axisand a proximal end portion and a distal end portion. A first Wheatstonebridge is disposed on a first face of the beam and includes first andsecond tension gauge resistors and first and second compression gaugeresistors. A second Wheatstone bridge is disposed on the first face ofthe beam and includes third and fourth tension gauge resistors and thirdand fourth compression gauge resistors. The first and third tensiongauge resistors and the first and third compression gauge resistors aredisposed at a proximal end portion of the beam. The second and fourthtension gauge resistors and the second and fourth compression gaugeresistors are disposed at a distal end portion of the beam.

In another aspect, a force sensor includes a beam having a longitudinalaxis and a proximal end portion and a distal end portion. A firsttension gauge half-bridge is disposed on a first face of the beam andincludes first and second tension gauge resistors. A second tensiongauge half-bridge is disposed on the first face of the beam and includesthird and fourth tension gauge resistors. A compression gaugehalf-bridge is disposed on the first face of the beam and includes firstand second compression gauge resistors. The first and third tensiongauge resistors and the first compression gauge resistor are disposed ata proximal end portion of the beam. The second and fourth tension gaugeresistors and the second compression gauge resistor are disposed at adistal end portion of the beam.

In yet another aspect, a force sensor includes a beam having alongitudinal axis and a proximal end portion and a distal end portion. Afirst bridge circuit is disposed on a first face of the beam andincludes multiple tension gauge resistors and at least one compressiongauge resistor. A second bridge circuit is disposed on the first face ofthe beam and includes multiple tension gauge resistors and at least onecompression gauge resistor. The at least one tension resistor from eachof the first and second bridge circuits and at least one compressiongauge resistor from one of the first and second bridges are disposed ata proximal end portion of the beam. The at least one tension resistorfrom each of the first and second bridge circuits and at least onecompression gauge resistor from one of the other of the first and secondbridges are disposed at a distal end portion of the beam.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 is an illustrative drawing representing a force sensor thatincludes a rectangular beam with four full-Wheatstone bridges(full-bridges).

FIG. 2 is an illustrative side elevation view of a distal portion of asurgical instrument with an elongated shaft having a force sensor beammounted thereon, in accordance with some examples.

FIG. 3 is an illustrative perspective view of a force sensor beam havinga beam side face with a pair of split bridge circuits formed thereon.

FIG. 4A is an illustrative schematic diagram representative of a firstfull-bridge bridge circuit.

FIG. 4B is an illustrative schematic diagram representative of a secondfull-bridge circuit.

FIG. 5 is an illustrative perspective view of a force sensor beam havinga beam side face with a pair of staggered split bridge circuits formedthereon.

FIG. 6 is an illustrative perspective view of a force sensor beam havinga beam face side with a pair of interleaved split half-bridge circuits.

FIG. 7 is an illustrative schematic diagram representative of threehalf-bridge circuits.

FIG. 8 is an illustrative side view of the beam showing arrangement ofthe gauge sensors of the interleaved three half-bridges example of FIG.6.

FIG. 9 is an illustrative perspective view of a force sensor beam havinga beam face with a pair of staggered split half-bridge circuits.

FIG. 10 is an illustrative side view representing arrangement of thegauge sensors of the staggered three half-bridges example of FIG. 9.

FIG. 11 is an illustrative perspective view of a beam with a T-gauge(tension) and C gauge (compression) disposed along a neutral axis of abeam.

FIG. 12 is an illustrative perspective view of a beam with tensiongauges R1 and R2 disposed along a neutral axis of a beam, respectively,at a proximal end portion and at a distal end portion of the beam.

FIG. 13 is an illustrative schematic diagram representing a half-bridgecircuit containing R1 and R2 of FIG. 12.

FIG. 14 is an illustrative side view of a force applied to a cantileverbeam indicating strain measurement at a distance 1 from the locationwhere force is applied to the beam.

FIG. 15 is an illustrative side view of a force applied to a cantileverbeam indicating strain measurement at a proximal distance and at adistal distance from the location where force is applied to the beam.

FIG. 16 is a side view of a cantilever beam.

FIG. 17A is a side view of a beam with a Wheatstone bridge disposed at aproximal end thereof.

FIG. 17B is a schematic diagram of the Wheatstone bridge circuit of FIG.17A.

FIG. 18 is a side view of a beam with a split Wheatstone bridge having atension gauge resistor R1 and a compression gauge resistor R2.

FIG. 19 is an illustrative drawing representing an example force sensorthat includes a beam with two split full bridge circuits thereon.

FIG. 20A is an illustrative simplified block diagram showingmechanically isolated lead lines extending between strain gauge elementsof two proximal split bridge circuit halves and strain gauge elements oftwo distal split bridge circuit halves.

FIG. 20B is an illustrative drawing showing certain details of a firstexample arrangement of the strain gauge elements of the example sensorof FIG. 20A.

FIG. 20C is an illustrative drawing showing certain details of a secondexample arrangement of the strain gauge elements of the example sensorof FIG. 20A.

FIG. 21 is an illustrative drawing showing a set of proximal connectionpads and a set of distal connection pads in which input pads andmeasurement pads have matching areas.

FIG. 22 is an illustrative drawing showing a set of proximal connectionpads and a set of distal connection pads in which measurement pads havelarger area than input pads.

FIG. 23 is an illustrative circuit schematic showing a force sensor thatincludes example first and second split bridge circuits disposed upon abeam that share excitation voltage inputs.

FIG. 24 is an illustrative drawing representing the tap lead lines forthe example dual split bridge circuit of FIG. 23.

DETAILED DESCRIPTION

view of a distal portion of a surgical instrument 202 with an elongatedshaft 204, shown in partially cut-way, having a force sensor beam 206mounted thereon, in accordance with some examples. The surgicalinstrument 202 includes an end effector 208, which may includearticulatable jaws, for example. During a surgical procedure, the endeffector 208 contacts anatomical tissue, which may result in X, Y, or Zdirection forces and that may result in moment forces such as a momentMY about a y-direction axis. The force sensor beam 206, which includes alongitudinal axis 210, may be used to measure X and Y forcesperpendicular to the longitudinal axis 210.

FIG. 3 is an illustrative perspective view of a force sensor beam 206having a beam side face 212 with a pair of split bridge circuits formedthereon. The bridge circuits are split in that a portion of each bridgecircuit is disposed at a proximal end portion of the beam and a portionof the bridge circuit is disposed at a distal end portion of the beam.More particularly, in the example of FIG. 3, the bridge circuits areconfigured as interleaved split full-Wheatstone bridges (full-bridges)formed thereon, having strain gauge resistors R_(C1), R_(C3), R_(T1),R_(T3) aligned along a neutral axis (equidistant from the sides of thebeam) at a proximal beam end portion 206P and having strain gaugeresistors R_(C2), R_(C4), R_(T2), R_(T4) aligned along a neutral axis ata distal beam end portion 206D, in accordance with some examples. Anidentical pair of interleaved split full-bridges (not shown) is formedon an adjacent orthogonal beam side face 214. The pair of interleavedsplit full-bridges formed on adjacent orthogonal side faces areconfigured to measure forces perpendicular to a longitudinal axis 210 ofthe beam 206, which may impart tensile strain to the beam. It will beappreciated that alignment of the gauge resistors along the neutral axisreduces the effect of off axis load since the neutral axis isinsensitive to off axis force and moments

FIG. 4A is an illustrative schematic diagram representative of a firstsplit full-bridge bridge of the pair, which includes a tension-gaugehalf-bridge comprising tension strain gauge resistors R_(T1) and R_(T2)and includes a compression-gauge half-bridge comprising compressionstrain gauge resistors R_(C1) and R_(C2), coupled as shown to receive aninput voltage V_(in) and to produce a first output voltage V_(O1). FIG.4B is an illustrative schematic diagram representative of a second splitfull-bridge of the pair, which includes a tension-gauge half-bridgetension strain gauge resistors R_(T3) and R_(T4) and includes acompression-gauge half-bridge comprising compression strain gaugeresistors R_(C3) and R_(C4), coupled as shown to receive the inputvoltage V_(in) and to produce a second output voltage V_(O2).

As explained more fully below, each tension strain gauge resistorR_(T1)-R_(T4) and each compression strain gauge resistor R_(C1)-R_(C4)includes a plurality of elongated resistor portions aligned in paralleland coupled end-to-end to form a serpentine or snake-like configuration.The elongated portions of the compression strain gauge resistorsR_(C1)-R_(C4) may be aligned perpendicular to the longitudinal axis 210of the beam to sense compression strain upon the beam 206. The elongatedportions of the tension strain gauge resistors R_(T1)-R_(T4) may bealigned parallel to the longitudinal axis of the beam to sense tensionstrain upon the beam.

Referring again to FIG. 3, each interleaved split full-bridge has atension gauge sensor resistor and a compression gauge sensor resistordisposed at the proximal end portion 206P of the beam 206. Eachinterleaved split full-bridge also has a tension gauge sensor and acompression gauge sensor disposed at the distal end portion 206D of thebeam 206. Specifically, strain gauge resistors, R_(T1), R_(T3) R_(C1)and R_(C3) are disposed at the proximal end portion 206P of the beam206, and strain gauge resistors R_(T2), R_(T4), R_(C2) and R_(C4) aredisposed at the distal end portion 206D of the beam 206. Moreover,elongated portions of strain gauge resistor R_(T1) are interleaved withelongated portions of strain gauge resistor R_(T3) so as to beco-located and occupy same longitudinal region of the beam. Elongatedportions of strain gauge resistor R_(C1) are interleaved with elongatedportions of strain gauge resistor R_(C3) so as to be co-located andoccupy same longitudinal region of the beam. The interleaved compressiongauge resistors R_(C1), R_(C3) are disposed closer to the proximal endof the beam than the interleaved tension resistors R_(T1), R_(T3).Elongated portions of strain gauge resistor R_(T2) are interleaved withelongated portions of strain gauge resistor R_(T4) so as to beco-located and occupy same longitudinal region of the beam. Elongatedportions of strain gauge resistor R_(C3) are interleaved with elongatedportions of strain gauge resistor R_(C4) so as to be co-located andoccupy same longitudinal region of the beam. The interleaved compressiongauge resistors R_(C2), R_(C4) are disposed farther from the distal endof the beam than the interleaved tension resistors R_(T2), R_(T4). Theinterleaving of strain gauge resistors of the example of FIG. 3 will bebetter understood from the description with reference to the interleavedhalf-bridge example represented in the illustrative drawings of FIG. 8.

FIG. 5 is an illustrative perspective view of a force sensor beam 206having a beam side face 212 with staggered split full-bridges formedthereon, each having strain gauge resistors R_(C1), R_(C3), R_(T1),R_(T3) aligned along a neutral axis at a proximal beam end portion 206Pand each having strain gauge resistors R_(C2), R_(C4), R_(T2), R_(T4)aligned along a neutral axis at a distal beam end portion 206D, inaccordance with some examples. The beam has four elongated rectangularside faces and rectangular end faces. An identical pair of staggeredsplit full-bridges (not shown) is formed on an adjacent orthogonal beamside face 214. The pair of staggered split full-bridges formed onadjacent orthogonal side faces are configured to measure forcesperpendicular to the longitudinal axis of the beam, which may imparttensile strain to the beam.

The illustrative schematic diagrams of FIGS. 4A-4B, described above, arerepresentative of first and second full-bridges of the staggered splitfull-bridges of FIG. 5. Each staggered split full-bridge has a tensiongauge sensor resistor and a compression gauge sensor resistor disposedat the proximal end portion 206P of the beam 206. Each full-bridge alsohas a tension gauge sensor and a compression gauge sensor disposed atthe distal end portion 206D of the beam 206. Specifically, strain gaugeresistors, R_(T1), R_(T3) R_(C1) and R_(C3) are disposed at the proximalend portion 206P of the beam 206, and strain gauge resistors R_(T2),R_(T4), R_(C2) and R_(C4) are disposed at the distal end portion 206D ofthe beam 206. However, unlike the pair of the interleaved splitfull-bridges of FIG. 3, the split staggered split bridges of FIG. 5 aredisposed in a “staggered” arrangement in which pairs of strain gauges ofeach full-bridge are disposed adjacent to each other at opposite endportions of the beam. Strain gauge resistors R_(T1) and R_(C1) of thefirst staggered split full-bridge are disposed adjacent to each other atthe proximal end portion 206P of the beam 206. Strain gauge resistorsR_(T3) and R_(C3) of the second staggered split full-bridge are disposedadjacent to each other at the proximal end portion 206P of the beam 206,offset farther from the proximal end portion 206B than are the straingauge resistors R_(T1) and R_(C1). Similarly, strain gauge resistorsR_(T2) and R_(C2) of the first staggered split full-bridge are disposedadjacent to each other at the distal end portion 206D of the beam 206.Strain gauge resistors R_(T4) and R_(C4) of the second staggered splitfull-bridge are disposed adjacent to each other at the distal endportion 206D, offset closer to the distal end portion 206D than are thestrain gauge resistors R_(T2) and R_(C2).

Both the interleaved split full-bridge and the staggered splitfull-bridge produce redundant first and second output voltages V_(O1)and V_(O2). The longitudinal distribution of the tension and compressiongauge sensor resistors of the pair of interleaved split full-bridges ofFIG. 3 and the pair of staggered split full-bridges of FIG. 5 cancelsout and substantially removes noise that may be caused by forces fromother sources such as off axis load moments in any of three directionsand forces in either of the two orthogonal directions to the force beingmeasured. Furthermore, each of the full-bridges of the pair ofinterleaved split full-bridges and each of the full-bridges of the pairof staggered split full-bridges includes both tension and compressionstrain gauge resistors configured to substantially cancel the effects oftemperature variation. Moreover, the interleaving of these strain gaugeresistors in the example of FIG. 3 ensures that they measure strain atthe same locations of the beam, which ensures that the redundant firstoutput voltage V_(O1) and second output voltage V_(O2) have matchingvalues even if there are temperature variations along the length of thebeam.

Thus, the distribution of gauge sensor resistors of each full-bridgebetween proximal and distal end portions of the beam removes the effectsof noise resulting from other sources of force and cancel the effects oftemperature so that the first and second output signals V_(O1) andV_(O2) produce the same redundant value. The first and second outputsignal values V_(O1) and V_(O2) may be compared to determine whetherthey have different values. A determination that V_(O1) and V_(O2) havedifferent values provides an indication of an error due to damage to theforce sensor, for example.

FIG. 6 is an illustrative perspective view of a force sensor beam 206having a beam face 212 with a pair of interleaved split half-Wheatstonebridges (half-bridges) each comprising tensile gauge sensor resistorsformed at proximal and distal end portions of the beam 206 and having ashared half-bridge comprising compression gauge resistors formed atproximal and distal end portions of the beam, in accordance with someexamples. A first tensile gauge half-bridge includes strain gaugeresistor R_(T1) at the proximal end portion 206P and includes straingauge resistor R_(T2) at the distal end portion 206D. A second tensilegauge half-bridge includes strain gauge resistor R_(T3) at the proximalend portion 206P and includes strain gauge resistor R_(T4) at the distalend portion 206D. The compression gauge half-bridge includes straingauge resistor R_(C1) at the proximal end portion and includes straingauge resistor R_(C2) at the distal end portion 206D. An identical pairof interleaved split half-bridges (not shown) comprising tensile gaugesensor resistors and an identical shared half-bridge (not shown)comprising compression gauge sensor resistors are formed on an adjacentorthogonal beam face 214. The pair of interleaved split half-bridgesformed on adjacent orthogonal faces are configured to measure forcesperpendicular to the longitudinal axis 210 of the beam 206, which mayimpart tensile strain to the beam. The shared compression gauge sensorresistors formed on adjacent orthogonal beam faces are configured tomeasure forces parallel to the longitudinal axis of the beam, which mayimpart compression strain to the beam 206.

FIG. 7 is an illustrative schematic diagram representative of threehalf-bridges. A first tension gauge half-bridge 602 includes tensionstrain gauge resistors R_(T1) and R_(T2) aligned along a neutral axis. Asecond tension gauge half-bridge 604 includes tension strain gaugeresistors R_(T3) and R_(T4) aligned along a neutral axis. A compressionhalf-bridge 606 includes compression strain gauge resistors R_(C1) andR_(C2) aligned along a neutral axis. Each of the first and secondtension gauge half-bridges 602, 604 and the compression gaugehalf-bridge 606 is coupled to receive an input voltage V_(in). The firsttension gauge half-bridge 602 is coupled to provide a first tensionforce output V_(O1). The second tension gauge half-bridge 604 is coupledto provide a second tension force output V_(O3). The compression gaugehalf-bridge 606 is coupled to produce a compression force output V_(O2).Each resistor R_(T1)-R_(T4) and R_(C1)-R_(C2) includes a plurality ofparallel elongated portions aligned in parallel and coupled end-to-endto form a serpentine or snake-like configuration. The elongated portionsof resistors R_(C1)-R_(C2) are aligned perpendicular to the longitudinalaxis 210 of the beam 206 to act as compression gauge sensors. Theelongated portions of resistors R_(T1)-R_(T4) are aligned parallel tothe longitudinal axis 210 of the beam 206 to act as tension gaugesensors.

Referring again to FIG. 6, each interleaved split half-bridge has atension gauge sensor resistor and a shared compression gauge sensorresistor disposed at the proximal end portion 206P of the beam 206. Eachinterleaved split half-bridge also has a tension gauge sensor and ashared compression gauge sensor disposed at the distal end portion 206Dof the beam 206. Specifically, strain gauge resistors, R_(T1), R_(C1)and R_(T3) are disposed at the proximal end portion 206P of the beam206, and strain gauge resistors R_(T2), R_(C2) and R_(T4) are disposedat the distal end portion 206D of the beam 206.

FIG. 8 is an illustrative side view of the beam, indicated by dashedlines, representing arrangement of the gauge sensors of the interleavedthree half-bridges example of FIG. 6. The elongated portions L_(T1) ofstrain gauge resistor R_(T1) are interleaved with elongated portionsL_(T3) of strain gauge resistor R_(T3) so as to be co-located and occupysame longitudinal region of the beam. The interleaved strain gaugeresistors R_(T1) and R_(T3) are farther from the proximal end of thebeam than is the compression gauge resistor R_(C1). Lead_(T1,T2), whichextends between the proximal and distal end portions, couples R_(T1)located at the proximal end portion 206P with R_(T2) located at thedistal end portion 206D. Elongated portions L_(T2) of strain gaugeresistor R_(T2) are interleaved with elongated portions L_(T4) of straingauge resistor R_(T4) so as to be co-located and occupy samelongitudinal region of the beam. The interleaved strain gauge resistorsR_(T2) and R_(T4) are disposed closer to the distal end of the beam thanis the compression gauge resistor R_(C2). Lead_(T3,T4), which extendsbetween the proximal and distal end portions, couples R_(T3) located atthe proximal end portion 206P with R_(T4) located at the distal endportion 206D. Lead_(C1,C2), which extends between the proximal anddistal end portions, couples R_(C1) located at the proximal end portion206P with R_(C2) located at the distal end portion 206D.

FIG. 9 is an illustrative perspective view of a force sensor beam 206having a beam face 212 with a pair of staggered split half-bridges eachcomprising tension gauge sensor resistors formed at proximal and distalend portions of the beam and having a shared half-bridge comprisingcompression gauge resistors formed at proximal and distal end portionsof the beam, in accordance with some examples. The schematic diagram ofFIG. 7 also is representative of staggered three half-bridges of FIG. 9.A first tensile gauge half-bridge includes strain gauge resistor R_(T1)at the proximal end portion 206P and includes strain gauge resistorR_(T2) at the distal end portion 206D. A second tensile gaugehalf-bridge includes strain gauge resistor R_(T3) at the proximal endportion 206P and includes strain gauge resistor R_(T4) at the distal endportion 206D. The compression gauge half-bridge includes strain gaugeresistor R_(C1) at the proximal end portion and includes strain gaugeresistor R_(C2) at the distal end portion 206D. An identical pair ofinterleaved split half-bridges (not shown) comprising tensile gaugesensor resistors and an identical shared half-bridge (not shown)comprising compression gauge sensor resistors are formed on an adjacentorthogonal beam face 214. The pair of interleaved split half-bridgesformed on adjacent orthogonal faces are configured to measure forcesperpendicular to the longitudinal axis 210 of the beam 206, which mayimpart tensile strain to the beam 206. The shared compression gaugesensor resistors formed on adjacent orthogonal beam faces are configuredto measure forces parallel to the longitudinal axis 210 of the beam,which may impart compression strain to the beam 206.

FIG. 10 is an illustrative side view representing arrangement of thegauge sensors of the staggered three half-bridges example of FIG. 9. Afirst tension gauge resistor half-bridge includes tension strain gaugeresistors, R_(T1) and R_(T2) aligned along a neutral axis. A secondtension gauge resistor half-bridge includes tension strain gaugeresistors, R_(T3) and R_(T4) aligned along a neutral axis. A compressiongauge resistor half-bridge includes compression strain gauge resistorsR_(C1) and R_(C2) aligned along a neutral axis. A first compressiongauge resistor R_(C1) is disposed at a proximal end portion of the beambetween first and third tension gauge resistors R_(T1), R_(T3), with thefirst tension gauge resistor R_(T1) disposed closer to the proximal endof the beam than the third tension gauge resistor R_(T3). A secondcompression gauge resistor R_(C2) is disposed at a distal end portion ofthe beam between second and fourth tension gauge resistors R_(T2),R_(T4), with the fourth tension gauge resistor R_(T4) disposed closer tothe distal end of the beam than the second tension gauge resistorR_(T2). Lead_(T1,T2), which extends between the proximal and distal endportions, couples R_(T1) located at the proximal end portion 206P withR_(T2) located at the distal end portion 206D. Lead_(T3,T4), whichextends between the proximal and distal end portions, couples R_(T3)located at the proximal end portion 206P with R_(T4) located at thedistal end portion 206D. Lead_(C1,C2), which extends between theproximal and distal end portions, couples R_(C1) located at the proximalend portion 206P with R_(C2) located at the distal end portion 206D.

Thus, both the interleaved split half-bridge and the staggered splithalf-bridge produce redundant first and second tension force outputvoltages V_(O1) and V_(O3). The longitudinal distribution of the tensionand compression gauge sensor resistors of the pair of interleaved splithalf-bridges of FIG. 6 and the pair of staggered split half-bridges ofFIG. 9 cancels out and substantially removes noise that may be caused byforces from other sources such as off axis load moments in any of threedirections and forces in either of the two orthogonal directions to theforce being measured. Moreover, in the interleaved half-bridges exampleof FIG. 6, the interleaving of the strain gauge resistors ensures thatthey measure strain at the same locations of the beam, which ensuresthat the redundant first output voltage V_(O1) and third output voltageV_(O3) have matching values even if there are temperature variationsalong the length of the beam.

In arriving at this invention, the inventor realized that eachhalf-bridge (both tension gauge half-bridges and compression gaugehalf-bridge) cancels the following stresses: moment, off axis force, offaxis moment, compression/tension, torsion, and ambient temperature. Theinventor also realized that each individual half-bridge output includesthe following: force and gradient temperature. The inventor furtherrealized that if V_(O) is the output of the half bridge

$V_{o} = {GF*\frac{V_{in}}{4}\left( {\epsilon_{p} - \epsilon_{d}} \right)}$

Where ε_(p) and ε_(d) represent the proximate stress and the distalstress and GF represents the gauge factor.

Then subtraction in (ε_(p)−ε_(d)) in the above equation cancels: moment,off axis force, off axis moment, compression/tension, torsion, andambient temperature, Now subtracting the ‘T gauge’ half-bridge outputand ‘C gauge’ half-bridge output, we cancel the following: gradienttemperature.

If Vo1 is the output of the half-bridges of ‘T gauges’ and Vo2 is theoutput of the half-bridge of ‘C gauges’, then the subtraction in Vo1−Vo2cancels ambient temperature and gradient temperature and the finaloutput after subtraction is only the sought after ‘Force’

For the split half-bridge examples of FIG. 6 and FIG. 9, redundantcomparisons may be performed as follows.

The redundant comparison is done after extracting the force signal andtemperature differential signal from the half-bridge measurementsV_(O1), V_(O2) and V_(O3).

For X axis, for example, (indicated by ‘x’)

We can use V_(O1_x) and V_(O2_x) to get Force1_x and deltaT1_x

We can use V_(O3_x) & V_(O2_x) to get Force1_x and deltaT2_x

For Y axis (indicated by ‘y’)

We can use V_(O1_y) & V_(O2_y) to get Force1_y and deltaT1_y

We can use V_(O2_y) & V_(O2_y) to get Force2_y and deltaT2_y

For redundancy check

Force x: we check whether Force1_x and Force1_x match

Force y: we check whether Force1_y and Force2_y match

deltaT: we check whether deltaT1_x and deltaT1_y match

These comparison checks will cover failures in any of the gauges.

deltaT2_x and deltaT2_y are kind of throw away terms because they don'tprovide additional information.

The math used to compute force and deltaT from voltage output is asfollows

$F_{} = \frac{{T\;{gauge}\; V_{o}} - {C\;{gauge}\; V_{o}}}{\left( {1 + \rho_{r}} \right)K_{f}}$${{where}\mspace{14mu} K_{f}} = {\frac{1}{4}{GFV}_{in}r\frac{l_{1} - l_{2}}{E\; 1}}$${\Delta\; T} = \frac{{C\;{gauge}\; V_{o}} + {\rho_{r}\left( {T\;{gauge}\; V_{o}} \right)}}{\left( {1 + \rho_{r}} \right)K_{T}}$${{where}\mspace{14mu} K_{T}} = {\frac{1}{4}{GFV}_{in}{CTE}}$

Where ‘T gauge V_(O)’ is V_(O1) and ‘C gauge V_(O)’ is V_(O2).

Thus, the distribution of gauge sensor resistors of each tension gaugehalf-bridge between proximal and distal end portions of the beam removesthe effects of noise resulting from other sources of force and cancelthe effects of temperature so that the first and second tension gaugeforce output signals V_(O1) and V_(O3) produce the same redundant value.The first and second tension gauge force output signal values VO1 andV_(O3) may be compared to determine whether they have different values.A determination that V_(O1) and V_(O3) have different values provides anindication of an error due to damage to the force sensor, for example.

Redundant temperature values are provided by the compression outputvalues Vo1 produced by the compression gauge half-bridges formedadjacent orthogonal faces. The compression output values Vo1 produced bythe half-bridges formed adjacent orthogonal faces may be compared todetermine whether they have different values. A determination that thecompression gauge half-bridges on adjacent orthogonal faces havedifferent values provides an indication of an error due to damage to theforce sensor, for example.

Proof

A. Determining T-Gauge Strain and C-Gauge Strain:

FIG. 11 is an illustrative perspective view of a beam with a T-gauge(tension) and C gauge (compression) disposed along a neutral axis of abeam.

Values used to determine force and change in temperature include:

F_(g): Force along the sensing plane

l: Location of the force applied

M: Moment applied perpendicular to the sensing plane

F_(Z): Force applied parallel to the neutral axis

I: Moment of inertia

A: Area or cross section

CTE: Coefficient of thermal expansion

ΔT: Change in temperature

ε: Strain

ρ: Poisson's ratio

T-gauge strain measurement equation:

$\epsilon = {{- \frac{F_{g}{lr}}{E\; 1}} + \frac{M_{\bot}r}{E\; 1} - \frac{F_{2}}{EA} + {{{CTE} \cdot \Delta}\; T}}$

C gauge strain measurement equation:

$\epsilon = {{- {{pr}\left( {{- \frac{F_{g}{lr}}{E\; 1}} + \frac{M_{\bot}r}{E\; 1} - \frac{F_{2}}{EA}} \right)}} + {{{CTE} \cdot \Delta}\; T}}$

B. Determining Force and ΔT:

FIG. 12 is an illustrative perspective view of a beam with tensiongauges R1 and R2 disposed along a neutral axis of a beam, respectively,at a proximal end portion and at a distal end portion of the beam. FIG.13 is an illustrative schematic diagram representing a half-bridgecircuit containing R1 and R2 of FIG. 12.

For the half-bridge of FIGS. 12-13, using the above T-gauge strainmeasurement, we can calculate V_(o) based upon R1, R2 and V_(in)

$V_{o} = {\frac{R\; 2}{{R\; 1} + {R\; 2}}*V_{in}}$R_(i) = R + Δ R_(i)  where  i = 1, 2 Δ R_(i) = GF*∈_(i)

The nominal resistance of both gauges is the same R value under no load.The change in resistance dependence on strain experienced by the gaugetimes the gauge factor. For small resistance change, using first orderapproximation we get the following equation.

$V_{o} = {\frac{1}{2} + {\frac{1}{4}\left( {\in_{2}{- \in_{1}}} \right)*{GF}*V_{in}}}$

We can substitute the strain equation to get V_(o) in terms of force.

${T\mspace{14mu}{gauge}\mspace{14mu} V_{o}} = {\frac{1}{2} + {\frac{1}{4}\left( {{F_{}r\frac{\left( {l_{1} - l_{2}} \right)}{El}} + {{CTE}\left( {{\Delta\; T_{2}} - {\Delta\; T_{1}}} \right)}} \right)*{GF}*V_{in}}}$

Similarly, we get an equation for the half-bridge using the compression(C) gauges.

${C\mspace{14mu}{gauge}\mspace{14mu} V_{o}} = {\frac{1}{2} + {\frac{1}{4}\left( {{{- \rho_{r}}F_{}r\frac{\left( {l_{1} - l_{2}} \right)}{El}} + {{CTE}\left( {{\Delta\; T_{2}} - {\Delta\; T_{1}}} \right)}} \right)*{GF}*V_{in}}}$

With equations for C gauge and T gauge, we can solve for F_(∥) andΔT=(ΔT₁−ΔT₂), we get,

$F_{} = \frac{{T\mspace{14mu}{gauge}\mspace{14mu} V_{o}} - {C\mspace{14mu}{gauge}\mspace{14mu} V_{o}}}{\left( {1 + \rho_{r}} \right)K_{f}}$${{where}\mspace{14mu} K_{f}} = {\frac{1}{4}{GFV}_{in}r\frac{l_{1} - l_{2}}{El}}$${\Delta\; T} = \frac{{C\mspace{14mu}{gauge}\mspace{14mu} V_{o}} - {\rho_{r}\left( {T\mspace{14mu}{gauge}\mspace{14mu} V_{o}} \right)}}{\left( {1 + \rho_{r}} \right)K_{f}}$${{where}\mspace{14mu} K_{r}} = {\frac{1}{4}{GFV}_{in}{CTE}}$

C. Basic Force Measurement Using Strain:

FIG. 14 is an illustrative side view of a force applied to a cantileverbeam indicating strain measurement at a distance 1 from the locationwhere force is applied to the beam.

Strain Equation for a perpendicular force applied on distal end aCantilever Beam is

$\epsilon = \frac{FLr}{EI}$Where

E: Modulus of Elasticity I: Moment of Inertia

We can see that the strain equation depends on the force ‘F’ applied aswell as the distance ‘1’ from the sensing point. Therefore, to be ableto measure the Force applied we need a second measurement to eliminatethe dependence of ‘1’. The most obvious way to do this is to measure thestrain at different point along the beam.

FIG. 15 is an illustrative side view of a force applied to a cantileverbeam indicating strain measurement at a proximal distance 1prox and at adistal distance 1dist from the location where force is applied to thebeam. Then we get

$\epsilon_{prox} = \frac{{Fl}_{prox}r}{EI}$$\epsilon_{dist} = \frac{{Fl}_{dist}r}{EI}$

Then when we subtract the 2 measurements we get

${\epsilon_{prox} - \epsilon_{dist}} = {\frac{Fr}{EI}\left( {l_{prox} - l_{dist}} \right)}$

The difference in distance is a known quantity; it is the distancebetween the two sensing points.

D. Force Measurements Under Noise Sources

FIG. 16 is a side view of a cantilever beam.

In typical force measurement scenario there is presence of noisesources/signals that are of no interest to us but still produce ameasurable strain on the beam, which we are sensing and this will resultin incorrect estimation of the force applied. Some other sources ofstrain that could be present are

-   -   Force in the 2 orthogonal directions    -   Moments in all 3 directions    -   Temperature changes

If the reference frame is selected such that the Force we want tomeasure is oriented along X axis then, the unwanted strain sources formeasuring are, Forces (Fy, Fz), Moments (Mx, My, Mz) and Temperature(ΔT)

Therefore, the most general strain equation for a point sensing elementon the cantilever beam oriented parallel to neutral axis is as follows

$\begin{matrix}{{T\mspace{14mu}{gauge}\mspace{14mu}{strain}} = \epsilon_{t}} \\{= {{- \frac{F_{x}{lr}}{EI}} - \frac{F_{y}{dr}}{EI} + \frac{F_{Z}}{EA} - \frac{M_{y}r}{EI} + \frac{M_{x}d}{EI} +}} \\{\epsilon_{t_{M_{z}}} + {{CTE}*\Delta\; T}}\end{matrix}$ $\begin{matrix}{{C\mspace{14mu}{gauge}\mspace{14mu}{strain}} = \epsilon_{c}} \\{= {{- {\rho_{r}\left( {{- \frac{F_{x}{lr}}{EI}} - \frac{F_{y}{dr}}{EI} + \frac{F_{Z}}{EA} - \frac{M_{y}r}{EI} + \frac{M_{x}d}{EI}} \right)}} +}} \\{\epsilon_{c_{M_{z}}} + {{CTE}*\Delta\; T}}\end{matrix}$WhereE: Modulus of elasticityI: Moment of inertiar: Perpendicular distance to YZ plane passing through the neutral axisd: Perpendicular distance to XZ plane passing through the neutral axisA: Area of cross sectionCTE: Coefficient of thermal expansion

ɛ_(c_(M_(z)))Strain due to torsion M_(Z) perpendicular to neutral axis

ϵ_(t_(M_(z)))Strain due to torsion M_(Z) parallel to neutral axisρ_(r): poisson ratio of the substrate

As can be see from the equation above, a point measurement of strainwill be dependent on lot of strain sources. The act of subtracting theproximal and distal measurements will also eliminate some sources ofstrain as common mode. The following gets eliminated

-   -   Force: Fz    -   Moments: My, Mz

If the sensing point/element is placed symmetrically about the neutralaxis (d=0), then the following does not affect the strain measurement

-   -   Force: Fy    -   Moment: Mx

Therefore the strain source that is not compensated is temperaturechange. The most trivial way to compensate for temperature is to useWheatstone bridge configuration using ‘C’ (compression) and ‘T’(tension) gauges to locally eliminate strain from temperature change.

FIG. 17A is a side view of a beam with a Wheatstone bridge disposed at aproximal end thereof. FIG. 17B is a schematic diagram of the Wheatstonebridge circuit of FIG. 17A.

The output of the Wheatstone bridge is as follows

$V_{out} = {\left( {\frac{R_{1}}{R_{1} + R_{2}} - \frac{R_{4}}{R_{3} + R_{4}}} \right)V_{in}}$

The resistance strain relationship of gauges is as followsR=R _(o) +GF*∈*R _(o)

If the nominal resistance of all the gauges are same and if substitutethe strain—resistance relationship and perform a first orderapproximation we get the following equation

$V_{out} = {{GF}*\frac{V_{in}}{4}\left( {\epsilon_{2} - \epsilon_{1} - \left( {\epsilon_{3} - \epsilon_{4}} \right)} \right)}$

When Fx is applied, the above equation reduces to the formV _(out) =K*F*lWhere K is a scalar constant and l=Δl+(l+ρ_(r))l₁ and Δl=l₂−l₁

So, we get output signal proportional to applied force

When there is a temperature change then the strain seen by all gaugesare same and the output ends up being zero, which implies temperaturechanges are compensated locally.

Therefore, to measure the applied force in the sensing direction (xaxis) we need two Wheatstone bridge located near the proximal and distalend of the beam.

E. Measuring Force Using a Single Wheatstone Bridge

FIG. 18 is a side view of a beam with a split Wheatstone bridge having atension gauge resistor R1 and a compression gauge resistor R2 disposedalong a neutral axis at a proximal end portion thereof and having atension gauge resistor R3 and a compression gauge resistor R4 disposedalong a neutral axis at a distal end portion thereof. The schematicdiagram of FIG. 17B is applicable to the split bridge of FIG. 18.

If we look at the trivial design we locally compensate for temperature,then measure two signals and externally subtract the two signals to getForce applied, but the structure of Wheatstone bridge equation has theability subtract signals internally, so we can use this to ouradvantage. Also, we can arrange the gauges such that instead oftemperature being compensated locally we can compensate temperatureeffect globally.

The temperature change seen at each measuring point can be decomposedinto ambient temperature change which is same everywhere and temperaturedifference which is temperature delta between the ambient temperatureand the temperature at the point. The ambient temperature change ends upbeing a common mode and just the act of subtracting the proximal anddistal signal eliminates its effects.

In the above configuration, we get the output signal as

$V_{out} = {{GF}*\frac{V_{in}}{4}\left( {\left( {\epsilon_{1} - \epsilon_{2}} \right) - \left( {\epsilon_{3} - \epsilon_{4}} \right)} \right)}$

If there was not temperature difference then we can see that R1 and R2together can measure force applied and similarly R3 and R4 can measurethe force applied, since R1, R2 is Tension gauge and R3,R4 iscompression gauge their relationship with force applied varies by−ρ_(r). Therefore, the total equation will reduce to

$V_{out} = {{GF}*\frac{V_{in}}{4}\left( {{K*F_{x}} - \left( {{- \rho_{r}}K*F_{x}} \right)} \right)}$

-   -   Where K is a fixed constant

$V_{out} = {{GF}*\frac{V_{in}}{4}K*\left( {1 + \rho_{r}} \right)F_{x}}$

The same configuration for temperature difference has different result,R1 and R2 measure the effect of temperature difference between theproximal and distal gauges, similarly R3 and R4 measure the effect oftemperature difference. Even though the two pairs are different gaugetypes, they have the same sensitivity to strain due to temperaturechanges

$V_{out} = {{GF}*\frac{V_{in}}{4}\left( {{K*\Delta\; T} - \left( {K*\Delta\; T} \right)} \right)}$

-   -   Where K is a fixed constant        V _(out)=0

So, we can roughly consider the ‘C’ gauges as temperature compensationgauges and ‘T’ gauges as the measurement gauges.

FIG. 19 is an illustrative drawing representing a force sensor 1900 thatincludes a beam 1902 with two split full bridge circuits thereon. Thebeam 1902 includes a proximal end portion 1902P and a distal end portion1902D and a center portion therebetween. A first split bridge circuitincludes strain gauge resistors R_(C1), R_(C3), R_(T1), R_(T3). Thefirst bridge circuit is split, with compression strain gauge resistorR_(C1) and tension gauge resistor R_(T1) located at a proximal endportion of the beam and compression strain gauge resistor R_(C3) andtension gauge resistor R_(T3) located at a distal end portion of thebeam, R_(T3). A second split bridge circuit includes strain gaugeresistors R_(C2), R_(C4), R_(T2), R_(T4). The second bridge circuit issplit, with compression strain gauge resistor R_(C2) and tension gaugeresistor R_(T2) located at a proximal end portion of the beam andcompression strain gauge resistor R_(C4) and tension gauge resistorR_(T4) located at a distal end portion of the beam.

Electrically conductive measurement output lead lines L1-L4 and inputsignal lead lines L5-L6 disposed upon the beam, electrically couple thestrain gauge resistors as shown. Measurement output lead line L1 isdisposed to extend integral with the proximal, center and distalportions of the beam to electrically couple R_(T1) and R_(T3) at a nodeV_(O1) ⁻. Measurement output lead line L2 is disposed to extend integralwith the proximal, center and distal portions of the beam toelectrically couple R_(C1) and R_(C3) at a node V_(O1) ⁺. Measurementoutput lead line L3 is disposed to extend integral with the beam toelectrically couple R_(C2) and R_(C4) at a node V_(O2) ⁺. Measurementoutput lead line L4 is disposed to extend integral with the beam toelectrically couple R_(T2) and R_(T4) at a node V_(O2) ⁻. Input signallead line L5 extends within the proximal portion of the beam toelectrically couple R_(C1), R_(C2), R_(T1) and R_(T2) at a node V₁ ⁺.Input signal lead line L6 extends within the distal portion of the beamto electrically couple R_(C3), R_(C4), R_(T3) and R_(T4) at a node V₁ ⁻.The lead lines L1-L6 are disposed integral with and mechanically coupledto the beam.

Input tap nodes V₁ ⁺, coupled to R_(T1), R_(T3), R_(C1), R_(C3), and V₁⁻, coupled to R_(T2), R_(T4), R_(C2), R_(C4) act as input signal nodesthat are coupled to receive positive and negative polarities of an inputexcitation signal. Measurement tap nodes V_(O1) ⁻ and V_(O1) ⁺ at therespective junction of R_(T1), R_(T3) and the junction of R_(C1),R_(C3), act as output measurement signal nodes that are coupled toprovide output measurement signals indicative of strain. Likewise,measurement tap nodes V_(O2) ⁻ and V_(O2) ⁺ at the respective junctionof R_(T2), R_(T4) and the junction of R_(C2), R_(C4), act as outputmeasurement signal nodes that are coupled to provide output measurementsignals indicative of strain.

Since each split bridge includes both strain gauge resistors disposed ata proximal end portion of the beam and strain gauge resistors disposedat a distal end portion of the beam, the lead lines L1-L6 thatelectrically couple the resistors of each bridge are relatively long.For example, lead lines that couple the resistors of the first andsecond split bridges of FIG. 19 are relatively longer than lead linesused to couple resistors of the non-split bridge circuits of theillustrative bridge of FIG. 1 in which each bridge is located eitherentirely at a proximal end portion of the beam or at a distal endportion of the beam.

A split bridge circuit can introduce undesirable amounts of unbalancedlead resistance between the strain gauges because the gauges aredistributed at beam location across large distances relative to the sizeof the strain gauges. The lead resistance can affect the accuracy of thesensor measurements and introduce cross coupling and temperaturedependent offset.

More particularly the conductive lead lines are integrally formed uponthe beam through deposition and etching process that produce the straingauges. Preferably, the lead lines do not change electrical propertiesin response to forces imparted to a beam or to a change in temperature,for example. However, the conductive lead lines extending along the beamto couple the active resistive strain gauges at opposite end portions ofthe beam are subjected to strain as the beam deflects in response toforce imparted to the beam, which changes the resistance of theconductive lines. In some examples, the length of the lead lines may becomparable in magnitude to the overall length of the conductor lineportions that make up the of the serpentine-shaped resistive straingauge elements. A change in resistance of the electrical conductionlines can result in distortion of strain measurements since theelectrical connection lines are not intended as behave as activeresistive elements of the bridge circuits. Thus, there is a need toreduce lead resistances and balances what minimal lead resistanceremains left over.

FIG. 20A-20B are is illustrative drawing representing an example forcesensor 2000 that includes a beam 2002 having a neutral axis 2004 andthat includes two split full bridge circuits with split bridge halveseclectically coupled by measurement output lead lines L1′-L4′ andelectrically coupled input lead lines L5-L6 are mechanically isolatedfrom the beam 2002. FIG. 20A illustrates the strain gauge elementsR_(C1)-R_(C4) and R_(T1)-R_(T4) of split bridge halves in simplifiedblock diagram form, and shows mechanically isolated measurement outputlead lines L1′-L4′ extending between the strain gauge elements of thesplit bridge halves and shows input signal lead line L5′ electricallycoupling R_(C1), R_(C2), R_(T1) and R_(T2) at a node V₁ ⁺ and showsinput signal lead line L6′ electrically coupling R_(C3), R_(C4), R_(T3)and R_(T4) at a node V₁ ⁻. FIG. 20B illustrates details of a firstexample arrangement of the strain gauge elements R_(C1)-R_(C4) andR_(T1)-R_(T4) of the example sensor of FIG. 20A. FIG. 20C illustratesdetails of a second example arrangement of the strain gauge elementsR_(C1)-R_(C4) and R_(T1)-R_(T4) of the example sensor 2000 of FIG. 20A.To simplify the drawings, the mechanically isolated measurement outputlead lines L1′-L4′ and input lead lines L5-L6 are omitted from FIGS.20B-20C, although their interconnections are explained below.

Referring to FIG. 20A, mechanically isolated measurement output leadlines L1′-L4′ of FIG. 20A correspond to correspondingly labeled integralmeasurement output lead lines L1-L4 of FIG. 19. The mechanicallyisolated lead lines L1′-L4′ are routed externally off the beam, oneexample of this implementation is to have the leads L1′-L6′ be part of aflex circuit/cable that is wirebonded to the beam. The flex cable willbe mounted such that it has enough slack that the deflection of the beamdue to force applied doesn't produce any strain the flex circuit/cable,moreover the lead lines on the flex circuit can be made of lowerresistance electrical material, such as Copper in comparison to thehigher resistance thin film material, such as Nichrome (an alloy ofNickel and Chromium), thus providing a lowerer magnitude the leadresistances. Thus, the lead lines L1′-L4′ that electrically couple thestrain gauge elements are mechanically isolated from strain impartedduring deflection of the beam 2002 due to a deflecting force (not shown)applied to the beam.

Referring to the example two-bridge arrangement FIG. 20B, except for thechange in lead lines, the arrangement of the example force sensor 2000of FIG. 20B is identical to that of the sensor of FIG. 19. Elongatedmeasurement output lead lines L1′-L4′ and input lead lines L5-L6 aremechanically isolated from the beam and shorter integral proximal leadline segments X_(P1)-X_(P6) and shorter integral distal lead linesegments X_(D1)-X_(D6) are disposed integral with the beam. As a result,less stress is imparted to the leads during mechanical deflection of thebeam, and therefore, less strain measurement distortion is incurred whencompared with the example sensor of FIG. 19.

Mechanically isolated lead line L1′ is disposed to extend in mechanicalisolation from the beam between the proximal, center and distal portionsof the beam, to electrically couple R_(C1) and R_(C3) at nodes labeledV_(O1) ⁻ in the proximal and distal bridge halves. Isolated lead lineL2′ is disposed to extend in mechanical isolation from the beam betweenthe proximal, center and distal portions of the beam, to electricallycouple R_(T1) and R_(T3) at nodes labeled V_(O1) ⁺ in the proximal anddistal bridge halves. Isolated lead line L3′ is disposed to extend inmechanical isolation from the beam between the proximal, center anddistal portions of the beam, to electrically couple R_(C2) and R_(C4) atnodes labeled V_(O1) ⁻ in the proximal and distal bridge halves.Isolated lead line L4′ is disposed to extend in mechanical isolationfrom the beam between the proximal, center and distal portions of thebeam, to electrically couple R_(T2) and R_(T4) at nodes labeled V_(O2) ⁺in the proximal and distal bridge halves. Proximal lead line elementsX_(P3)-X_(P4) are coupled to voltage V₁ ⁺ to thereby electrically coupleR_(C1), R_(C2), R_(T1) and R_(T2) at the voltage V₁ ⁺. Distal lead lineelements X_(D3)-X_(D4) are coupled to voltage V₁ ⁻ to therebyelectrically couple R_(C3), R_(C4), R_(T3) and R_(T4) to the voltage V₁⁺.

Referring to the example two-bridge arrangement FIG. 20C, each of thecompression gauge resistors and each of the tension gauge resistors isarranged symmetrically about the neutral axis 2004 of the beam.Moreover, in the second example arrangement, R_(C1) and R_(T1) of theproximal half of the first bridge and R_(C2) and R_(T2) of the proximalhalf of the second bridge are arranged symmetrically about a proximaltransverse axis 2006P between them at the proximal portion of the beam.R_(T1) partially surrounds R_(C1), and R_(T2) partially surroundsR_(C2). Terminals of R_(C1) and R_(T1) are transversely alignedproximate to the proximal transverse axis 2006P, and terminals of R_(C2)and R_(T2) are transversely aligned distal to the proximal transverseaxis 2006P. Likewise, R_(C3) and R_(T3) of the distal half of the firstbridge and R_(C4) and R_(T4) of the distal half of the second bridge arefurther arranged symmetrically about a distal transverse axis 2006Dbetween them at the distal portion of the beam. Terminals of R_(C3) andR_(T3) are transversely aligned proximate to the distal transverse axis2006D, and terminals of R_(C4) and R_(T4) are transversely aligneddistal to the distal transverse axis 2006D. R_(C3) partially surroundsR_(C3), and R_(T4) partially surrounds R_(C4).

Still referring to FIG. 20C, elongated lead line segments L1′-L4′ aremechanically isolated from the beam and shorter integral proximal leadline segments X_(P7)-X_(P8) and shorter integral distal lead linesegments X_(P7)-X_(P8) are disposed integral with the beam. As a result,less stress is imparted to the leads during mechanical deflection of thebeam, and therefore, less strain measurement distortion is incurred whencompared with the example sensor of FIG. 19.

The coupling between nodes and lead lines is the same for the first andsecond example arrangements of FIGS. 20B-20C. More specifically,referring to FIG. 20C, mechanically isolated lead line L1′ is disposedto extend in mechanical isolation from the beam between the proximal,center and distal portions of the beam, to electrically couple R_(C1)and R_(C3) at nodes labeled V_(O1) ⁻ in the proximal and distal bridgehalves. Isolated lead line L2′ is disposed to extend in mechanicalisolation from the beam between the proximal, center and distal portionsof the beam, to electrically couple R_(T1) and R_(T3) at nodes labeledV_(O1) ⁺ in the proximal and distal bridge halves. Isolated lead lineL3′ is disposed to extend in mechanical isolation from the beam betweenthe proximal, center and distal portions of the beam, to electricallycouple R_(C2) and R_(C4) at nodes labeled V_(O1) ⁻ in the proximal anddistal bridge halves. Isolated lead line L4′ is disposed to extend inmechanical isolation from the beam between the proximal, center anddistal portions of the beam, to electrically couple R_(T2) and R_(T4) atnodes labeled V_(O2) ⁺ in the proximal and distal bridge halves.Proximal lead line elements X_(P7)-XPs are coupled to voltage V₁ ⁺ tothereby electrically couple R_(C1), R_(C2), R_(T1) and R_(T2) at thevoltage V₁ ⁺. Distal lead line elements X_(D7)-X_(D8) are coupled tovoltage V₁ ⁻ to thereby electrically couple R_(C3), R_(C4), R_(T3) andR_(T4) to the voltage V₁ ⁺.

FIG. 21 is an illustrative drawing showing a set of proximal connectionpads 2110P and a set of distal connection pads 2110D disposed upon thebeam 2002 of FIG. 20A in which input pads 2112 and measurement pads 2114have matching areas. The set of proximal connection pads 2110P and theset of distal connection pads 2110D are disposed upon an example beam2002 for electrical coupling of mechanically isolated measurement outputlead lines L1′-L4′ and input signal lines L5′-L6′ to the strain gaugeresistors. Wire bonds (they are there in FIG. 21 it is the small arcgoing from the pads to the collection of lead lines. Also please includeL5-L6 to bundle of leads) electrically couple the measurement outputleads L1′-L4′ to the pads measurement pads 2114 and to couple the inputsignal leads L5′-L6′ to the input pads 2112. Input signal pads 2112 arecoupled to receive the excitation voltage V₁ ⁺, V₁ ⁻, as shown.Measurement output pads 2114 are coupled to receive the sensed outputvoltages V_(O1) ⁻, V_(O1) ⁺, V_(O2) ⁻, V_(O2) ⁺, as shown.

Conventionally, all pads typically are the same area, and therefore,have the same resistance. The wire bonds can be variable, however, whichcan impact the resistance of the electrical connection between the padsand the leads, which in turn, can influence zero offset between signalson the different electrical leads and also may influence temperaturesensitivity of the electrical connection. Increased resistance at theinput pads affects gain of the voltage, which is related to sensitivityof measurement. Increased resistance at the output measurement padsaffects the zero offset. In general, gain can be more easily managedthan zero offset, using software, for example.

FIG. 22 is an illustrative drawing showing a set of proximal connectionpads 2110P′ and a set of distal connection pads 2110D′ in whichmeasurement output pads 2114′ have larger area than input signal pads2112′. Input pad area is sacrificed to provide larger measurement padarea. The larger measurement pad area results in reduced resistance ofthe measurement pads 2114′, which reduces the impact of wire bondvariation upon connection pad resistance, which in turn, reduces theimpact of wire bond variation upon zero offset. Thus, a relativeincrease in measurement pad size compared with input signal pad sizereduces variability of measurement pad resistance reducing the impact ofvariation in location and size of a wire bond upon measurement accuracy.

Still referring to FIG. 22, the set of proximal connection pads 2110P′and the set of distal connection pads 2110D′ are disposed upon anexample beam 2002 for electrical coupling of mechanically isolated leadlines L1′-L4′ and L5′-L6′ to strain gauge resistors. Wire bondselectrically couple the leads L1′-L4′ to the measurement output pads2114′. Input signal pads 2112′ are coupled to receive the excitationvoltage V₁ ⁺, V₁ ⁻, as shown. Measurement signal output pads 2114′ arecoupled to receive the sensed output voltages V_(O1) ⁻, V_(O1) ⁺, V_(O2)⁻, V_(O2) ⁺, as shown.

FIG. 23 is an illustrative circuit schematic showing a force sensor 2000that includes example first and second split bridge circuits 2310, 2320disposed upon a beam that share excitation voltage inputs V₁ ⁺, V₁ ⁻.Half of each bridge circuit is disposed at a proximal end portion of thebeam, and half of each bridge circuit is disposed at a distal portion ofthe beam. To simplify the drawing, however, FIG. 23 does not show thephysical separation of the proximal and distal halves at opposite endportions of the beam. FIG. 23 shows the example arrangement of the firstand second split bridge circuits at non-overlapping separate locationsupon the beam. FIG. 5 shows an example staggered arrangement of thefirst and second split bridge circuits. FIG. 6 shows an exampleinterleaved arrangement of the first and second split bridge circuits.

As explained above, measurement redundancy is achieved using two splitbridge circuits, each having a half-bridge disposed at the proximal endportion of the beam and having a half-bridge portion disposed at thedistal end portion of the beam. For example, a mismatch of correspondingoutput measurements of the two split bridge circuits is indicative ofmeasurement error and possible damage of one or both of the two bridgecircuits. To ensure accuracy in determination of measurement mismatches,for example, the resistance of corresponding tap lead lines for firstand second bridge circuits should match.

FIG. 24 is an illustrative drawing representing the tap lead lines forthe example dual split bridge circuit of FIG. 23. Tap lead lines betweena voltage level and a circuit node have equal lead line lengths and haveuniform line widths to provide matching resistance between a given tappoint and each of the circuit nodes electrically coupled to the tappoint. Thus, resistances between each tap point and the nodeselectrically coupled to the tap point is balanced.

In particular, for example, the tap lead line length and lead line widthbetween tap point of excitation input voltage V₁ ⁺ and the circuit nodereceiving V₁ ^(+A) equals the tap lead line length and lead line widthbetween tap point of voltage V₁ ^(+B) and circuit node receiving V₁^(+B) as indicated by the single hash marks on either side of the V₁ ⁺tap point.

The tap lead line length and lead line width between tap point ofexcitation input voltage V₁ ⁻ and the circuit node receiving V₁ ^(−A)equals the tap lead line length and lead line width between tap point ofvoltage V₁ ⁻ and circuit node receiving V₁ ^(−B) as indicated by thedouble hash marks on either side of the V₁ ⁻ tap point.

The tap lead line length and lead line width between tap point ofmeasurement voltage V_(O1) ⁺ and the circuit node providing V_(O1) ^(+A)equals the tap lead line length and lead line width between tap point ofvoltage V_(O1) ⁺ and circuit node receiving V_(O1) ^(+B) as indicated bythe three hash marks on either side of the V_(O1) ⁺ tap point.

The tap lead line length and lead line width between tap point ofmeasurement voltage V_(O1) ⁻ and the circuit node providing V_(O1) ^(−A)equals the tap lead line length and lead line width between tap point ofvoltage V_(O1) ⁻ and circuit node receiving V_(O1) ^(−B) as indicated bythe four hash marks on either side of the V_(O1) ⁻ tap point.

The tap lead line length and lead line width between tap point ofmeasurement voltage V_(O2) ⁺ and the circuit node providing V_(O2) ^(+A)equals the tap lead line length and lead line width between tap point ofvoltage V_(O2) ⁺ and circuit node receiving V_(O2) ^(+B) as indicated bythe five hash marks on either side of the V_(O2) ⁺ tap point.

The tap lead line length and lead line width between tap point ofmeasurement voltage V_(O1) ⁻ and the circuit node providing V_(O2) ^(−A)equals the tap lead line length and lead line width between tap point ofvoltage V_(O1) ⁻ and circuit node receiving V_(O2) ^(−B) as indicated bythe five hash marks on either side of the V_(O2) ⁻ tap point.

The equal tap lead line length ensures balanced tap line lead lengthresistance on each side of each tap point resulting in more accuratedeterminations of circuit damage based upon measurement output signalmismatches, for example.

Although illustrative examples have been shown and described, a widerange of modification, change and substitution is contemplated in theforegoing disclosure and in some instances, some features of theexamples may be employed without a corresponding use of other features.One of ordinary skill in the art would recognize many variations,alternatives, and modifications. Thus, the scope of the disclosureshould be limited only by the following claims, and it is appropriatethat the claims be construed broadly and in a manner consistent with thescope of the examples disclosed herein. The above description ispresented to enable any person skilled in the art to create and use aforce sensor with a beam and a distributed bridge circuit. Variousmodifications to the examples will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother examples and applications without departing from the scope of theinvention. In the preceding description, numerous details are set forthfor the purpose of explanation. However, one of ordinary skill in theart will realize that the invention might be practiced without the useof these specific details. In other instances, well-known processes areshown in block diagram form in order not to obscure the description ofthe invention with unnecessary detail. Identical reference numerals maybe used to represent different views of the same or similar item indifferent drawings. Thus, the foregoing description and drawings ofexamples in accordance with the present invention are merelyillustrative of the principles of the invention. Therefore, it will beunderstood that various modifications can be made to the examples bythose skilled in the art without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. A force sensor comprising: a beam comprising aproximal end portion, a distal end portion, a first face between theproximal end portion of the beam and the distal end portion of the beam,a longitudinal axis extending between the proximal end portion of thebeam and the distal end portion of the beam, and a neutral axisextending in the first face of the beam parallel to the longitudinalaxis and equidistant from opposite edges of the first face; a firstWheatstone bridge on the first face of the beam, the first Wheatstonebridge comprising first and second tension gauge resistors and first andsecond compression gauge resistors; and a second Wheatstone bridge onthe first face of the beam, the second Wheatstone bridge comprisingthird and fourth tension gauge resistors and third and fourthcompression gauge resistors; wherein the first and third tension gaugeresistors and the first and third compression gauge resistors are at theproximal end portion of the beam; wherein the second and fourth tensiongauge resistors and the second and fourth compression gauge resistorsare at the distal end portion of the beam; and wherein the first,second, third, and fourth tension gauge resistors are each located onthe first face and are each aligned along the neutral axis in the firstface of the beam.
 2. The force sensor of claim 1, wherein: the first andthird tension gauge resistors are co-located on the beam; the first andthird compression gauge resistors are co-located on the beam; the secondand fourth tension gauge resistors are co-located on the beam; and thesecond and fourth compression gauge resistors are co-located on thebeam.
 3. The force sensor of claim 1, wherein: the first and thirdtension gauge resistors comprise corresponding first and third tensiongauge resistor elongated portions, and the first and third tension gaugeresistor elongated portions are interleaved with one another; the firstand third compression gauge resistors comprise corresponding first andthird compression gauge resistor elongated portions, and the first andthird compression gauge resistor elongated portions are interleaved withone another; the second and fourth tension gauge resistors comprisecorresponding second and fourth tension gauge resistor elongatedportions, and the second and fourth tension gauge resistor elongatedportions are interleaved with one another; and the second and fourthcompression gauge resistors comprise corresponding second and fourthcompression gauge resistor elongated portions, and the second and fourthcompression gauge resistor elongated portions are interleaved with oneanother.
 4. The force sensor of claim 1, wherein: the first and thirdtension gauge resistors comprise corresponding first and third tensiongauge resistor serpentine configurations, and the first and thirdtension gauge resistor serpentine configurations are interleaved witheach other; the first and third compression gauge resistors comprisecorresponding first and third compression gauge resistor serpentineconfigurations, and the first and third compression gauge resistorserpentine configurations are interleaved with each other; the secondand fourth tension gauge resistors comprise corresponding second andfourth tension gauge resistor serpentine configurations, and the secondand fourth tension gauge resistor serpentine configurations areinterleaved with each other; and the second and fourth compression gaugeresistors comprise corresponding second and fourth compression gaugeresistor serpentine configurations, and the second and fourthcompression gauge resistor serpentine configurations are interleavedwith each other.
 5. The force sensor of claim 1, wherein: the first,second, third, and fourth compression gauge resistors are each locatedon the first face and are each aligned along the neutral axis in thefirst face of the beam.
 6. The force sensor of claim 1, wherein: one ofthe first and third tension gauge resistors is between the first andthird compression gauge resistors, and the other of the first and thirdtension gauge resistors is adjacent to only one or the other of thefirst and third compression gauge resistors; and one of the second andfourth tension gauge resistors is between the second and fourthcompression gauge resistors, and the other of the second and fourthtension gauge resistors is adjacent to only one or the other of thesecond and fourth compression gauge resistors.
 7. The force sensor ofclaim 6, wherein: the first, second, third, and fourth compression gaugeresistors are each located on the first face and are each aligned alongthe neutral axis in the first face of the beam.
 8. The force sensor ofclaim 1, wherein: the beam comprises a second face orthogonal to thefirst face; the force sensor comprises a third Wheatstone bridge on thesecond face of the beam, and the third Wheatstone bridge comprises fifthand sixth tension gauge resistors and fifth and sixth compression gaugeresistors; the force sensor comprises a fourth Wheatstone bridge on thesecond face of the beam, and the fourth Wheatstone bridge comprisesseventh and eighth tension gauge resistors and seventh and eighthcompression gauge resistors; the fifth and seventh tension gaugeresistors and the fifth and seventh compression gauge resistors are atthe proximal end portion of the beam; and the sixth and eighth tensiongauge resistors and the sixth and eighth compression gauge resistors areat the distal end portion of the beam.
 9. A force sensor comprising: abeam comprising a proximal end portion, a distal end portion, a firstface between the proximal end portion of the beam and the distal endportion of the beam, a longitudinal axis extending between the proximalend portion of the beam and the distal end portion of the beam, and aneutral axis extending in the first face of the beam parallel to thelongitudinal axis and equidistant from opposite edges of the first face;a first tension gauge half Wheatstone bridge on the first face of thebeam, the first tension gauge half Wheatstone bridge comprising firstand second tension gauge resistors; a second tension gauge halfWheatstone bridge on the first face of the beam, the second tensiongauge half Wheatstone bridge comprising third and fourth tension gaugeresistors; and a compression gauge half Wheatstone bridge on the firstface of the beam, the compression gauge half Wheatstone bridgecomprising first and second compression gauge resistors; wherein thefirst and third tension gauge resistors and the first compression gaugeresistor are at the proximal end portion of the beam; wherein the secondand fourth tension gauge resistors and the second compression gaugeresistor are at the distal end portion of the beam; and wherein thefirst, second, third, and fourth tension gauge resistors are eachlocated on the first face and are each aligned along the neutral axis inthe first face of the beam.
 10. The force sensor of claim 9, wherein:the first and third tension gauge resistors are co-located on the beam;and the second and fourth tension gauge resistors are co-located on thebeam.
 11. The force sensor of claim 9, wherein: the first and thirdtension gauge resistors comprise corresponding first and third tensiongauge resistor elongated portions, and the first and third tension gaugeresistor elongated portions are interleaved with one another; and thesecond and fourth tension gauge resistors comprise corresponding secondand fourth tension gauge resistor elongated portions, and the second andfourth tension gauge resistor elongated portions are interleaved withone another.
 12. The force sensor of claim 9, wherein: the first andthird tension gauge resistors comprise corresponding first and thirdtension gauge resistor serpentine portions, and the corresponding firstand third tension gauge resistor portions are interleaved with eachother; and the second and fourth tension gauge resistors comprisecorresponding second and fourth tension gauge resistor serpentineportions, and the corresponding second and fourth tension gauge resistorportions are interleaved with each other.
 13. The force sensor of claim9, wherein: the first and second compression gauge resistors are eachlocated on the first face and are each aligned along the neutral axis inthe first face of the beam.
 14. The force sensor of claim 9, wherein:the first compression gauge resistor is between the first and thirdtension gauge resistors; and the second compression gauge resistor isbetween the second and fourth tension gauge resistors.
 15. The forcesensor of claim 14, wherein: the first and second compression gaugeresistors are each located on the first face and are each aligned alongthe neutral axis in the first face of the beam.
 16. The force sensor ofclaim 9, wherein: the beam comprises a second face orthogonal to thefirst face; the force sensor comprises a third tension gauge halfWheatstone bridge on the second face of the beam, and the third tensiongauge half Wheatstone bridge comprises fifth and sixth tension gaugeresistors; the force sensor comprises a fourth tension gauge halfWheatstone bridge on the second face of the beam, and the fourth tensiongauge half Wheatstone bridge comprises seventh and eighth tension gaugeresistors; the force sensor comprises a second compression gauge halfWheatstone bridge on the second face of the beam, and the secondcompression gauge half Wheatstone bridge comprises third and fourthcompression gauge resistors; the fifth and seventh tension gaugeresistors and the third compression gauge resistor are at a proximal endportion of the beam; and the sixth and eighth tension gauge resistorsand the fourth compression gauge resistor are at a distal end portion ofthe beam.
 17. A force sensor comprising: a proximal end portion, adistal end portion, a first face between the proximal end portion of abeam and the distal end portion of the beam, and a longitudinal axisextending between the proximal end portion of the beam and the distalend portion of the beam; a first bridge circuit on the first face of thebeam, the first bridge circuit comprising multiple tension gaugeresistors; a second bridge circuit on the first face of the beam, thesecond bridge circuit comprising multiple tension gauge resistors; and athird bridge circuit on the first face of the beam, the third bridgecircuit comprising multiple compression gauge resistors; wherein atension gauge resistor from each of the first and second bridge circuitsand a compression gauge resistor from the third bridge circuit are atthe proximal end portion of the beam; and wherein a tension gaugeresistor from each of the first and second bridge circuits and acompression gauge resistor from the third bridge circuit are at thedistal end portion of the beam.
 18. The force sensor of claim 17,wherein: the tension gauge resistor of the first bridge circuit at theproximal end portion of the beam is co-located on the beam with thetension gauge resistor from the second bridge circuit at the proximalend portion of the beam; and the tension gauge resistor of the firstbridge circuit at the distal end portion of the beam is co-located withthe tension gauge resistor from the second bridge circuit at the distalend portion of the beam.
 19. The force sensor of claim 17, wherein: thecompression gauge resistor from the third bridge at the proximal endportion of the beam is between the tension gauge resistors from each ofthe first and second bridge circuits at the proximal end portion of thebeam; and the compression gauge resistor from the third bridge at thedistal end portion of the beam is between the tension gauge resistorsfrom each of the first and second bridge circuits at the distal endportion of the beam.
 20. The force sensor of claim 17 embodied in asurgical instrument.
 21. A force sensor comprising: a beam comprising aproximal end portion, a distal end portion, a first face between theproximal end portion of the beam and the distal end portion of the beam,a longitudinal axis extending between the proximal end portion of thebeam and the distal end portion of the beam, and a neutral axisextending in the first face of the beam parallel to the longitudinalaxis and equidistant from opposite edges of the first face; a firstWheatstone bridge on the first face of the beam, the first Wheatstonebridge comprising first and second tension gauge resistors and first andsecond compression gauge resistors; and a second Wheatstone bridge onthe first face of the beam, the second Wheatstone bridge comprisingthird and fourth tension gauge resistors and third and fourthcompression gauge resistors; wherein the first and third tension gaugeresistors and the first and third compression gauge resistors are at theproximal end portion of the beam; wherein the second and fourth tensiongauge resistors and the second and fourth compression gauge resistorsare at the distal end portion of the beam; and wherein the first,second, third, and fourth compression gauge resistors are each locatedon the first face and are aligned along the neutral axis in the firstface of the beam.